OBJECTIVE: To characterize the physical and chemical events leading to the expression of radiation-induced damage at the cellular level. METHODS: The above objective has been approached mainly through the use of the spores of Bacillus megaterium as the test organism. Other systems have and will be used such as Phage T7 (used in the past) and transforming DNA (currently in use). Both X-rays and U.V. have been used. Pulse radiolysis studies at the Center for Fast Kinetics Research are now beginning to obtain chemical data relative to biological data. RESULTS: The relationship to water (H2O and D2O) and water structure to radiation sensitivity has been clarified through experiments in "dry" systems and damage in "dry" systems compartmentalized into several types whose characteristics have been defined. A proposed model of radiation sensitization in solution has been presented involving the OH radical and H2O2. The sensitizing action of many metal ions including Cu, Fe, Co, Pt and Ag has been studied biologically and chemistry studies relative to these biological results are now underway so as to understand the mechanisms involved in radiation sensitization. Mechanisms involving organic sensitizers such as PNAP and diacetyl have been studied. The effect of nitrate in radiation sensitization of the bacterial spore has been reported and the effects of nitrate concentration on radical yields chemically has been reported. Results have been obtained on the effects of radiation on transforming DNA and the effects of various O2 concentrations, alcohol scavengers and the presence of Ag ion and Cu on sensitization of transforming DNA have been established. Pulse radiolysis studies on nucleotides and intact DNA in solution are in progress.